1. Field of the Invention
The present invention relates to an electrophotographic apparatus such as a printer and a copying machine adopting an electrophotographic process.
In an electrophotographic apparatus, the surface resistance of a photosensitive drum is decreased by moisture adsorption of the surface of the photosensitive drum to cause the occurrence of smear. The present invention relates to a technique for efficiently preventing such smear.
2. Description of the Related Art
An electrophotographic printer is constructed so that means for charging, exposing, developing, transferring, separating, cleaning, erasing, etc. are arranged around a photosensitive drum. The surface of the photosensitive drum is uniformly charged by corona discharge, and is then exposed to light corresponding to characters or the like to be printed, thereby forming an electrostatic latent image on the photosensitive drum. The electrostatic latent image is developed with a toner (developer) to form a toner image on the photosensitive drum. Thereafter, a sheet of paper is supplied onto the toner image, and corona discharge is applied from the back side of the paper to thereby transfer the toner image formed on the photosensitive drum to the paper. Then, the paper sticking to the photosensitive drum is separated and the toner image transferred to the paper is fixed by heat, thus completing a cycle of printing. After the transfer step, the surface of the photosensitive drum is subjected to cleaning and erasing steps. Thereafter, the above series of steps are similarly repeated.
However, there is a problem such that smear occurs in the repeated use of the electrophotographic printer. The "smear" is a phenomenon that a printed image (character) is faint or the edge of a character is smudged and that when a print ratio is low, characters become illegible, whereas when the print ratio is high, they are blurred to become dark as a whole. This phenomenon termed smear occurs when the surface resistance of the photosensitive drum in its normal condition as shown in FIG. 1A is decreased by the cause to be hereinafter described to allow movement of charges around the photosensitive drum as shown in FIG. 1B.
The cause of decrease in the surface resistance of the photosensitive drum is classified into (1) deterioration of a surface layer (e.g., Se: selenium) of the photosensitive drum itself by long-term exposure of the photosensitive drum to ozone (O.sub.3), and (2) ozone exposure and absorption of moisture in the atmospheric air after formation of a filming layer on the surface of the photosensitive drum. The smear occurring in the field at present is almost caused by the absorption of moisture into the filming layer.
The filming layer to be formed on the surface of the photosensitive drum will now be described. A developing unit, a cleaner, and a sheet of paper come to contact with the photosensitive drum. In such contact areas between the photosensitive drum and the other members, ozone (O.sub.3) due to corona discharge acts on silica (Si) etc. in paper particles, fluorine (F) etc. in a carrier coating layer of a developer, and organic solvents and moisture in the atmospheric air as shown in FIG. 2 by the repeated use of the electrophotographic printer. As a result, a coating is formed on the surface of the photosensitive drum. This coating is called the filming layer, which is prone to absorb moisture, resulting in a decrease in the surface resistance of the photosensitive drum to cause the occurrence of smear.
As measures for preventing the smear, a technique as disclosed in Japanese Patent Laid-open No. Sho 59-208558 is known, for example. In this technique, a heat roller is brought into contact with a photosensitive drum to heat and dry the photosensitive drum at 40.degree. C. to 200.degree. C., thereby preventing a decrease in the surface resistance. Although the prior art technique has its own effect, it does not consider at all the relation between smear and humidity as one of factors of environment where the electrophotographic printer is installed. Accordingly, the smear cannot be efficiently prevented.
Further, as the photosensitive drum is heated continuously or periodically, a power consumption due to heating becomes large. In addition, the formation of the filming layer cannot be effectively prevented to shorten the lifetime of the photosensitive drum.